The Past as the Key to the Future

Reconstructing past sea levels on the Isles of Scilly, and projecting how the island landscape might change in the future.

The Scillonian archipelago of approximately 200 islands, islets, and rocks lies about 45 kilometres south-west of Land’s End. At present only five islands are inhabited: St Mary’s, St Agnes, St Martin’s, Bryher, and Tresco. Several small islands including Teän, Samson, and St Helen’s were occupied in the recent past. The islands are separated by wide expanses of shallow subtidal and intertidal environments, created by rising sea-levels during the 11,700 years of the Holocene.

The Lyonesse Project is a collaboration between the Cornwall Archaeology Unit, the Cornwall and Isles of Scilly Maritime Archaeology Society, the universities of Aberystwyth, Cardiff, and Exeter, and Historic England. It has investigated the changing nature of the Scilly environment during this period, reconstructing how the prehistoric inhabitants of the islands adapted to their changing land and seascape, as well as how the islands might change in the future.

The changing environment

Sediment samples from 25 locations across the archipelago were taken for palaeoenvironmental analysis. Research on this material included pollen analysis, to reconstruct vegetation change, and analysis of foraminifera: amoeba-like single-celled micro-organisms which tell us about changing types of saltmarsh environment. Optically Stimulated Luminescence of sand units and radiocarbon dating of organic material provided a chronological framework for interpreting such palaeoenvironmental proxies.

The pollen record shows that the Scillies were colonised rapidly by woodland at the beginning of the Holocene, reflecting the warming that took place across north-west Europe at this time. Open ground within the forested landscape of the Mesolithic would have been generated by a combination of fires and floods, the latter caused by rising sea-levels. The amount of woodland cover began to decline from about 5000 cal BC, with the vegetation cover showing much greater diversity than before. For the last 3,000 years the landscape of Scilly has largely been open with land used for grazing and cultivation.

Past sea-level index points

In order to build a record of sea-level change for the Isles of Scilly, samples were identified from which an accurate palaeo-elevation above sea-level (an ‘indicative meaning’) could be established. Foraminifera secrete tiny shells (tests) that survive as fossils, and these can pinpoint the elevation of the deposit in which they are found relative to the sea-level at the time of their formation. Sea-level index points were therefore derived from samples in cores containing foraminifera that were indicative of environmental conditions different to those of the present day.

A new sea-level curve for Scilly

The plotting of sea-level index points allows a reconstruction of the timing and tempo of changing sea-levels. The data show that previous reconstructions of sea-level change were inaccurate: an imaginative attempt by the late Professor Charles Thomas (1985) lacked any scientific dates and was based on the vertical elevation of submerged archaeological sites that could be broadly dated on the basis of associated material culture and place -name evidence. This analysis clearly overemphasised the rapidity with which the sea rose. The more gradual rise proposed by Radcliff and Straker (1996), based on the first radiocarbon dated samples for the Scillies, is more consistent with the new data – but only for the last few thousand years. The new data generated is more robust, more complete, more accurate – and correlates well with the most recent estimates of relative sea-level changes around the British Isles.

Sea-level curves for the Isles of Scilly

Sea-level rise and changes in land area

Sea level rise in Scilly would have had a significant impact on the prehistoric landscape and how it was used. The timing and nature of changing land areas, especially the process of separation of the individual islands, has been the subject of considerable speculation and debate. The new sea-level data provides a much more robust basis for reconstructing these changes, especially for the period from 5000 cal BC, when a larger number of secure sea-level index points are available.

The new sea-level curve was used to model a number of palaeo-shorelines, and thus to reconstruct past geographies. These reconstructions indicate a rapid rise in sea-level from about 5000 cal BC onwards. Scilly had been a continuous landmass - for example, in around 7000 cal BC – but now St Agnes and the other western islands began to separate from the rest.

By 3000 cal BC tidal flooding began to separate the elements of the main island group. Tresco, Bryher, and Samson remained joined throughout this period. The most dramatic loss of land took place from 2500/2000 cal BC, when the equivalent of two-thirds of the entire modern area of the islands began to be lost to the sea. By 1500 cal BC the configuration of the islands was approaching that of the present day.

Future sea-level rise

The provision of baseline data on local relative sea-level change has been used alongside estimates of past sea-levels to estimate future sea-level rise in Scilly. UK Climate Projections is a climate analysis tool which features the most comprehensive climate projections for the UK, shown in probabilistic form, and illustrating the level of confidence in each prediction.

‘The loss of future land surface will be significant for the character of Scilly’

Possible changes in land and intertidal areas by 2100 resulting from four scenarios for future sea-level rise were given in Lowe et al (2009) and suggest there will be some loss of land surface, but the largest change will be a reduction in the size of the intertidal zones.

The result of this loss will be significant for the character of Scilly, with the islands becoming permanently separated by deeper waters, and surrounded by individual, relatively narrow, intertidal areas. There is therefore a risk of flooding to low-lying and narrow areas of land, which could in turn lead to the formation of new islands. Although potentially dramatic, these changes, if they happen as predicted, will be relatively minor compared to those that took place in the second half of the third millennium cal BC.

The future extent of land and intertidal area under four different projections for sea-level rise by 2100.

Robust palaeoenvironmental reconstructions, such as those produced as part of the Lyonesse Project, are important if we are to meet the environmental challenges of the future. They help in determining policies for mitigating or adapting to climate change, and evaluating what can be learned from the past.

The authors

Charlie Johns, BA MCIfA

Archaeology Projects Officer, Cornwall Archaeological Unit

Charlie Johns has worked with the Cornwall Archaeological Unit for 26 years. An experienced field archaeologist, he is now responsible for developing the Unit’s maritime capacity and for projects in the Isles of Scilly. Past projects in Scilly have included the Bryher sword and mirror burial excavation, the Isles of Scilly Rapid Coastal Zone Assessment Survey (RCZAS), the Scilly Historic Environment Research Framework (SHERF) and the Lyonesse Project, a study of the historic coastal and marine environment of the archipelago. He helped to establish and continues to organise the islands’ Community Archaeology Group. Current projects include the South Cornwall RCZAS and delivering Heritage at Risk services in Scilly, both funded by Historic England.

Peter Marshall

Scientific Dating Coordinator with Historic England

Peter Marshall works in the Research Group as part of the Scientific Dating Team, coordinating the programme of commissioned radiocarbon dating. He has been involved in many iconic archaeological projects over the last twenty years, providing precise chronologies that help us to understand past human activities and how landscapes have changed.

Further information

Charman, D, Johns, C, Camidge, K, Marshall, P, Mills, S, Mulville, J and Roberts, H M 2016 The Lyonesse Project: A Study of the Evolution of the Coastal and Marine Environment of the Isles of Scilly. Truro: Cornwall County Council